C++ Program Error - Virtual destructors

Question

Possible Duplicate:
Pure virtual destructor in C++

class A{
    public:
        virtual ~A()=0;
};

class B:public A{
        int *i;
    public:
        ~B() {delete i;}
};

int main(){
    B* temp = new B;
}

I'm just trying to have B be an implementation of A. For some reason I cannot do this.

Note that right now, your `B::~B` has undefined behavior -- it deletes `i`, but `i` was never initialized. — Jerry Coffin, Nov 04 '12 at 02:40
You aren't dynamically allocating `i` anywhere, so the `delete i;` will crash. — j_random_hacker, Nov 04 '12 at 02:41
What do you mean "For some reason I cannot do this"? What happens when you try? What do you expect to happen instead? Where is the code where you attempt to use `B`? — Karl Knechtel, Nov 04 '12 at 02:44

PiotrNycz · Accepted Answer · 2012-11-04T02:45:47.633

3

In C++ destructor can be pure virtual:

class A{
    public:
        virtual ~A()=0;
};

But in every case it needs to be implemented:

inline A::~A() {}

Otherwise A is not usable class. I mean destruction of derived (S/B) is not possible. And possibility of destruction is needed in this line:

  B* temp = new B;

because in case of throwed exception - compiler will automatically destructs temp...

edited Nov 04 '12 at 02:45

answered Nov 04 '12 at 02:39

PiotrNycz

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Provide inline empty implementation of your virtual destructor in header file. There is no other way. Otherwise destruction of derived (S/B) is not possible. – PiotrNycz Nov 04 '12 at 02:43
thanks adding that line fixed my program. – user1202422 Nov 04 '12 at 02:48
I don't think the line with `B` actually can invoke any destructor, since if `new` throws it does so before completing the construction of `B` (actually before even starting it; it fails to get the raw memory), so the destructor `B::~B` will not be called. But this (a line calling for the _construction_ of a `B`) is as good a place as can be found for the compiler to warn you about the impossibility to destruct a `B`; either you will do that ultimately, or you've got a memory leak. – Marc van Leeuwen Jul 23 '14 at 12:37
@MarcvanLeeuwen An exception can be thrown from inside of `B::B()` after constructing its `A` part. So compiler needs to have access to implementation of `A::~A()`... I mean `B::~B() : A(), i(new int(13)) {}` In case of `bad_alloc` for `new int(13)` - `A::~A()` will be invoked... – PiotrNycz Jul 23 '14 at 14:57
@PiotrNycz: You are right, I overlooked that possibility. (And you meant `B::B()` where you wrote `B::~B()`; I'm sure you would never dream of calling `new` from a destructor.) – Marc van Leeuwen Jul 23 '14 at 16:13

score 1 · Answer 2 · answered Nov 04 '12 at 02:49

According to your comment "Yeah i want A to basically be just a container class. Do not want any implementation of A". Your class B shall protected/private inherited from A instead of public inherite. virtual ~A() is allowed to be pure but you still need to provide implementation to ~A().

class A{
public:
  virtual ~A() = 0
  {
    cout << "~A"<<endl;
  }
};

class B : private /*protected*/ A{
  int *i;
public:
  B() : A(){
    i = new int;
  }
  ~B() {
    delete i;
  }
};

int main(){
    B* temp = new B;
    delete temp;
    return 0;
}

C++ Program Error - Virtual destructors

2 Answers2